1. Technical Field
This invention relates to a novel apparatus utilizing fiber optics for colorimetric measurement of chemical properties. More particularly, this invention relates to a fiber optic probe which employs a confronting face optical gap measurement configuration while allowing an overall probe diameter sufficiently small to permit the probe to be inserted into living tissue directly or by prior insertion into a 16-gauge or smaller hypodermic needle.
2. Background Art
Colorimetric measurement of chemical properties is well known in the art. One simple example is the use of a phenophthalein solution which turns red in the presence of a base while becoming clear in the presence of an acid. The use of a colorimetric substance in combination with a fiber optics light source and detector has been taught by many references. Light supplied through a transmitting optical fiber is transmitted through a colorimetric substance mixed with a chemical whose properties are to be measured, and received by a receiving optical fiber which transmits that light to a light detector. A change in color of the colorimetric substance thus changes the light transmissivity of the mixture resulting a different amount of light measured by the light detector. The use of light for the measurement of such quantities as blood pH in vivo is superior to electrical measurement because of the resulting reduced irritation and shock hazard to living tissue. The optical fibers provide a means of channeling the light and making a measurement probe of convenient size.
In prior art configurations, a first optical fiber is connected at one end to a light transmitter and has its opposite end prepared by making a cut at 90.degree. to the axis of the fiber to form a face. A second optical fiber is connected to a light detector at one end and has a face prepared on its opposite end in a manner like that of the first optical fiber.
In one common configuration, the faces of the two optical fibers are arranged so as to confront each other, allowing light from the transmitting fiber to be directed through the chemical to be measured and directly into the face of the receiving optical fiber. The two faces are thus parallel and separated from one another by a distance of typically 0.01 in. so as to form an optical gap. In the simplest configuration of this type, the optical fibers may extend away from the optical gap with their respective axes coincident. A small and more manageable configuration is made by bending the optical fibers so that they may be arranged parallel to one another at a distance away from the optical gap. Such a configuration has been taught in U.S. Pat. No. 3,123,066 by Brumley.
Using the configuration as taught by Brumley, an optical probe may be constructed in which the body consists of two parallel optical fibers suitably fastened together to produce a relatively small diameter probe body. There has been thought to exist a fundamental lower limit to probe tip size, since the respective optical fibers must be bent away from the direction of the probe body direction near the tip and then bent back toward each other to permit the respective faces to closely confront each other at the optical gap. The fundamental lower limit in probe tip size results from the fact that there is a lower limit to the bending radius of the optical fiber. The literature of the fiber optics art teaches that an optical fiber exhibits dramatically reduced transmissivity when bent with a bending radius near that of its outer diameter.
A second configuration has been used which allows a smaller probe tip size. An example of this configuration is disclosed by Peterson, et al., in U.S. Pat. No. 4,200,110. The two optical fibers are arranged parallel to each other along the entire probe length. At the tip, the optical fiber faces are arranged so that they are generally parallel but face the same direction rather than confronting one another. In such a configuration, light is transmitted into the chemical to be measured, thence reflected back to be received at the face of the receiving optical fiber. Light reception then depends upon either the light scattering properties of the chemical to be measured, or upon placement of a reflector at the probe tip. While this second configuration does not require the bending of the optical fibers, it does result in a reduced amount of light available at the face of the receiving optical fiber.
Both configurations have a common disadvantage, in that the measurement chamber is located at the tip of the probe. This limits the sharpness of the probe. Also, there is a greater opportunity for tip breakage if the probe is inserted directly into living tissue. In the prior art, one way of protecting the probe tip has been to insert the probe into a hypodermic needle, and then insert the needle into living tissue. Placement of shielding material at the probe tip interferes with the introduction of the chemical to be colorimetrically measured into the measurement chamber.